The use of quaternary ammonium compounds as collectors in reverse froth flotation processes for calcite ores has long been known. The meaning of the term “reverse froth flotation” is that the froth is used for carrying the gangue mineral rather than carrying the valuable concentrate, i.e. the gangue is recovered in the froth product. See, for instance, U.S. Pat. No. 4,995,965, where calcium carbonate and impurities, such as silicate, are separated by floating the silicate and concentrating the calcium carbonate in the remainder, in the presence of collectors such as dialkyl dimethyl quaternary ammonium compounds. However, dialkyl quaternary products, such as products of formula (A) below, which are currently used for reversed flotation of calcite, have the drawback of being toxic for aqueous organisms and are also regarded as being not readily biodegradable in environment.

DE 19602856 proposes to use biodegradable ester quats as collectors in a reverse froth flotation process. These products are quaternary fatty acid alkanolamine ester salts. However, such ester quats were found to degrade, by hydrolysis and/or biologically during the flotation step, releasing fatty acid, particularly in the typical process where the aqueous phase is recycled. In the calcite reverse froth flotation process there is a risk that the fatty acid released may attach to the calcite and float the mineral, resulting in poor yields.
Recently a new class of oligomeric ester quats, such as products having formula (B) below, has been proposed in the patent application EP 1949963 A1.

These products meet the demand of being nontoxic, readily biodegradable products that seem to be sufficiently efficient in flotation. However, they exhibit the same disadvantage as the ester quats mentioned above, with fast release of fatty acid upon hydrolysis, especially when used in a flotation process environment of high pH (around 10) and an elevated temperature (above 30° C.). The release of fatty acid soap can possess a risk as this substance has the opposite collecting properties to the ester quats, thus supporting flotation of the valuable calcite which is then going to waste (see e.g. the comparison experiment in Example 5 of the present invention).
Hence there is a continued need to optimize and/or find alternatives for the reverse froth flotation process of calcium carbonate ores. In this respect it is particularly important that the amount of acid-insoluble material in the product is as low as possible, the yield of product is as high as possible, and that a product of high quality (particularly brightness) is obtained. It should be realized that reducing the amount of acid-insoluble material and increasing the yield are two mutually conflicting goals. More specifically, reducing the amount of acid-insoluble material is typically achieved by floating off a large amount of material, but this reduces the yield, and vice versa.